Trace element geochemistry of altered volcanic ash layers (tonsteins) in Late Permian coal-bearing formations of eastern Yunnan and western Guizhou Provinces, China

Trace element compositions were determined (by instrumental neutron activat ion analysis; INAA) in 30 samples of synsedimentary volcanic ash-derived to nsteins and detrital claystones from coal seams within the late Permian coa l-bearing formation of eastern Yunnan and western Guizhou Provinces, China. The characteristics of trace-element geochemistry in the tonsteins can be distinguished from those of detrital claystones because of the former's uni que volcanic-ash origin. The detrital claystones are characterized by their relatively high content of V, Ti, Sc, Cr, Co and Ni, relatively low conten t of Th and U, Th/U ratio, and small negative Eu anomaly (Eu/Eu* 0.63-0.93) . Overall, these trace element characteristics are consistent with a mafic source similar to the composition of basalt rooks in the erosional region o n the western edge of the study area. In contrast, the tonsteins are low in V, Ti, Sc, Cr, Co and Ni contents and have a high Th/U ratio with a distin ct negative Eu anomaly (Eu/Eu* normally in the range of 0.2-0.4), consisten t with a silicic magmatic source. Within the group of tonsteins, those from the lower section (P-2.1) of the coal-bearing formation are relatively high in Ti, Zr, Hf Nb, Ta and rare ea rth elements (REE), as compared to those from the middle and upper sections (P2.2+3). In trace-element discrimination diagrams (scatter plots) of Hf-T a, Ti-Ta, TI-V, Hf-Sc, Lu-Hf and Lu-Th, tonsteins from the P-2.1 horizon al ways fall in isolated distribution areas, separate from the tonsteins of th e P2.2+3 horizon. These results suggest that the source materials of tonste ins from the two separate horizons were probably derived from volcanic ash falls of two distinctly different natures. Based on a comparison of the con centrations and assemblages of trace elements between various magmatic rock s, the source materials of tonsteins from P-2.1 horizon were mostly compose d of calc-alkalic, silica-poor volcanic ash (similar to rhyodacitic magma), whereas those from P2.2+3 were apparently more siliceous and K-rich (rhyol itic magma). Thus, tonsteins from the two different horizons are characteri zed by unique geochemical properties, which remain constant over a wide lat eral extent. Integration of trace-elemental compositions with mineralogical and textural observations makes possible the establishment of tonstein str atigraphy, thus, facilitating more precise and reliable coal-seam correlati ons. (C) 2000 Elsevier Science B.V. All rights reserved.